Yea, I do not aware that!. Added criteria that answer will not be equal negative and -1 is used for invalid input. Thanks for your idea. Sorry for using 'undefined' which cause confusion.

True! In this kata, it is an over-simplified version of the whole process by just simply referring result that cannot conclude the result as undefined. In reality, more standard will be made to deal with that, thank you for your comments. Much appreciated.

I will change 'undefined' to -1, is that ok?, thanks

if they absorbs the same units of light, the concentration can't be determined as no matter what is the concentration, the units of absorbance will be the same. Thanks for the suggestion btw, i will correct it now, I am new in creating kata, hope i will make sth great at last. Thank you.

Thanks a lot for the suggestion. I am new to creating kata and also coding(lol). I haev made some corrections based on your suggestion. Hopefully, it will solve the aforementioned issue. Do you mind spare some time to check my work again? Thank you.

This test does not contain much sense for the rounding down part. As lets say if you calculate 1000.5 in the third year, it will be regard as >1000 as time is continuous. However, this is the requirement, you can just assume this is a test or a practice. Stop blaming it and said it is a broken test just because you don't look at the question carefully.Reading the requirement carefully is an important step in programming lol.

it is a problem in rounding down, it will be better for you to check the requirement. For 3, it will reach to an ans of 1214.228 which rounded down into 1214, hence not >1214. Yea, this does not make much sense, but this is the requirement.